Hitherto, a system that includes an engine, two rotating electric machines, and a power distribution mechanism (a planetary gear mechanism) is known as a hybrid system. In the hybrid system, a rotation shaft of the engine, a rotation shaft of the first rotating electric machine, a rotation shaft of the second rotating electric machine, and a drive wheel are connected to the rotation components of the power distribution mechanism. Patent Literature 1 below discloses a hybrid system in which a differential device including a pair of first and second planetary gear mechanisms, a clutch, and two brakes are interposed between a rotation shaft of an engine and a rotation component of a power distribution mechanism. The differential device is used as a transmission device which changes the rotation speed of the engine. The clutch has a configuration in which one engagement portion is connected to the rotation shaft of the engine and a carrier of the first planetary gear mechanism and the other engagement portion is connected to a ring gear of the first planetary gear mechanism. In the first planetary gear mechanism, the carrier and a sun gear respectively engage with the sun gear and the ring gear of the second planetary gear mechanism. The sun gear of the first planetary gear mechanism and the ring gear of the second planetary gear mechanism are connected to the carrier of the power distribution mechanism. The first brake is used to stop the rotation of the ring gear of the first planetary gear mechanism and the other engagement portion of the clutch. The second brake is used to stop the rotation of the carrier of the second planetary gear mechanism. In the hybrid system, an under-drive mode (a UD mode) is set in the middle-load state and the high-load state by the engagement of the clutch and the release of the brakes, an over-drive mode (an OD mode) is set in the low-load state by the release of the clutch and the second brake and the engagement of the first brake, and a backward movement mode is set by the release of the clutch and the first brake and the engagement of the second brake.